On Fri, 12 Sep 2014 11:03:19 +0200 David Brown <david.brown@xxxxxxxxxxxx> wrote: > On 12/09/14 02:46, Chris Murphy wrote: > > > > On Sep 11, 2014, at 5:38 PM, Brassow Jonathan <jbrassow@xxxxxxxxxx> > > wrote: > > > >> Neil (or anyone else), > >> > >> I know that trim/discard support was added back in 2012 (commit > >> 9db90880). However, I thought there were still issues regarding > >> what happens when various sync operations occur. I'd like to turn > >> on discard support in dm-raid.c (a oneline patch) if things are in > >> order. I can enable any, all or none depending on your > >> recommendation. (I assume RAID1/10 is easier than the parity > >> RAIDs.) > > > > If all the controller and drive support it then it should pass > > through, but there's the problem whether the SSD supports > > deterministic trim. If it doesn't, a check check > md/sync_action > > will report mismatches in md/mismatch_cnt; and a repair will probably > > corrupt the volume. So you can still use trim with a drive that > > returns non-deterministic results with raid0/1/10, but you can't rely > > on the result of md/mismatch_cnt and you can't do repair type > > scrubs. > > > > For raid5/6, it's a problem to use trim if the drive returns > > non-deterministically for trimmed blocks. I'd think that in addition > > to DRAT being supported, it'd need to support DZAT. > > > > smartctl --identify=wb /dev/diskX | grep -i trim > > > > > > Chris Murphy > > > > Would it be possible to change trim/discard commands into write zero > blocks for some SSDs? There is a BLKZEROOUT ioctl which writes zeros, using the 'WRITE SAME' SCSI command if possible. I suspect it would be quite easy to modify "fstrim" to use BLKZEROOUT instead of BLKDISCARD. NeilBrown > A number of SSD controllers support transparent > compression, so writing large batches of zeros will result in very small > writes to the actual flash, and the SSD controller will be able to > recycle flash used by the overwritten logical blocks just as if they > were trimmed. Obviously writing zeros will take longer in transfer than > trim commands, but the result on the disk would be similar and it would > be guaranteed deterministic. > > David > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
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